Burst gate pulse generator

ABSTRACT

A burst gate pulse generating circuit includes a switching transistor having an input coupled to a source of pulses representative of a horizontal sync component of a video signal which includes a burst interval, and having an output coupled to an LC resonant circuit with a predetermined LC time constant. The resonant circuit is excited into ringing when the transistor conducts in response to an input pulse, and a resulting ringing waveform is used in conjunction with the inverse conduction characteristic of the transistor to turn off the transistor prior to the completion of one full cycle of ringing. An output voltage pulse is provided which is coincident with the burst interval and which corresponds to the first full half cycle of one polarity of the ringing waveform.

This invention relates to pulse generating circuits and, moreparticularly, to such circuits for developing a gating signal toseparate a color synchronizing burst reference signal from chrominanceinformation of a composite color television video signal.

In a color television system such as the NTSC type adopted by the UnitedStates, for example, a composite color television video signal includesa chrominance (color) component with color information phase andamplitude modulated on a suppressed color subcarrier of 3.58 MHz, and aluminance (brightness) component. The video signal also includessynchronizing (sync) pulses occurring during a blanking interval, and acolor burst reference signal in synchronized relationship with the colorsubcarrier. The burst component is represented by several cycles of aknown phase of the subcarrier signal, and occurs shortly after ahorizontal sync pulse during the blanking interval.

In a color television receiver for this system, the burst component isseparated from the remainder of the video signal to provide a referencesignal of proper phase and frequency for demodulating the colorcomponent. It has been customary in separating the burst component toapply to an amplifier only those signal frequencies in the upper portionof the video frequency range containing the burst and chrominance signalfrequencies. By periodically gating the amplifier into conduction withgate pulses coincident with the burst interval of the video signal, theburst component is separated to the exclusion of the remainder of theapplied signal.

A suitably timed burst gate pulse can be derived from a horizontal lineflyback pulse produced by a horizontal deflection circuit of thetelevision receiver. The horizontal sync pulse itself can also be usedto derive the burst gate pulse, since the horizontal sync pulse isalready present in the video signal in a fixed time relationship withthe burst component. A burst gate pulse derived from a horizontal syncpulse may be preferred in some applications because the flyback pulse issusceptible to being misplaced relative to the burst interval byadjustment of, for example, the hold control in the horizontaldeflection circuit. Adjustment of the horizontal circuit via the holdcontrol may cause an undesirable change in the timing, amplitude orshape of the flyback pulse such that a portion of the flyback pulse mayundesirably occur in time coincidence with some of the video informationof the composite signal. In such a case, the gated amplifier will passnot only the burst component but also a portion of the videoinformation. A burst gate pulse derived from the horizontal sync pulseis not affected by adjustment of the horizontal circuit.

It is desirable that circuits employed for obtaining a burst gate pulsefrom either a horizontal flyback pulse or a horizontal sync pulse shouldbe relatively uncomplicated and inexpensive, and should provideaccurate, noise-immune operation.

A burst gate pulse generator according to the present inventioncomprises a source of operating potential, an input circuit forproviding a pulse representative of a horizontal synchronizing componentof a color television video signal, and a transistor switch having aninput electrode coupled to the input circuit, an output electrodecoupled to the source of operating potential, and a common electrode.The burst gate pulse generator also includes a resonant circuitcomprising at least an inductance and a capacitance. The resonantcircuit has a predetermined time constant and is coupled to the outputelectrode of the transistor. The resonant circuit is excited intoringing when the transistor conducts in response to the input pulse toproduce a ringing waveform with a period determined by the timeconstant. The resonant circuit coacts with an inverse conductioncharacteristic of the transistor to render the transistor non-conductiveprior to completion of a first full cycle of the waveform to provide anoutput pulse corresponding to a first full half cycle of one polarity ofthe waveform and coincident with a burst interval of the video signal.

FIG. 1 of the drawing illustrates a portion of a color televisionreceiver employing a circuit in accordance with the present invention.

FIGS. 2-8 show waveforms useful in understanding the operation of thecircuit shown in FIG. 1.

Referring to FIG. 1, a video signal processing unit 20 is responsive toradio frequency television signals received by an antenna 10. Videosignal processing unit 20 generates a composite video signal comprisingchrominance, luminance, sound and synchronizing components by means ofsuitable intermediate frequency amplifier and detector circuits (notshown).

An output of signal processing unit 20 is coupled to a synchronizing(sync) signal separator 30 for separating horizontal and vertical syncpulses from the video signal. Separated horizontal and vertical syncpulses are applied to deflection circuits of an image reproducingkinescope (not shown). Horizontal sync pulses (V_(s)) are also coupledfrom an output of sync separator 30 to an input of a burst gate pulsegenerator 50, which will be described in greater detail subsequently.Burst gate output pulses from circuit 50 are coupled to an input of aburst gate processing unit 70. Burst gate processing unit 70 serves toprovide a single burst gate pulse or push-pull burst gate pulses,according to the requirements of subsequent chroma processing circuits.

Video signals from signal processing unit 20 are also coupled to achroma bandpass filter 75. Filter 75 selectively passes the relativelyhigher frequency chrominance component of the video signal. Outputsignals from bandpass filter 75 and output burst gate pulses from unit70 are coupled to respective inputs of a chroma processing unit 80,which serves to derive color difference signals R-Y, B-Y and G-Y fromthe chrominance component of the video signal. The color differencesignals are coupled to a kinescope driver unit (not shown) where theyare matrixed with luminance (Y) signals conventionally derived from thevideo signal to produce red (R), blue (B) and green (G) color signalsfor driving a kinescope (not shown).

Burst gate pulse generator 50 includes an input voltage divider formedby a resistor 52 and a resistor 53, and an integrating networkcomprising a resistor 56 and a capacitor 58 coupled to a base of atransistor 60. Transistor 60 is arranged in common emitter configurationand may be of the commercial type MPS A20 marketed by MotorolaCorporation. A collector of transistor 60 is coupled to a source ofoperating voltage (+4 volts) via a load resistor 61. A resonant circuit65 including an inductor 68 and a capacitor 66 is coupled across acollector-emitter path of transistor 60. A burst gate output pulse(V_(O)) is developed by gating circuit 50 at the junction of capacitor66 and inductor 68.

In the operation of circuit 50, reference is first made to FIG. 7, whichillustrates a portion of a video signal showing the relative positionsof the horizontal sync pulse component and the burst component in thevideo signal. It should be recognized that the illustrated portion ofthe waveform occurs during an interval of the order of ten microsecondsand recurs at the line scanning rate. In FIG. 7, the horizontal synccomponent comprises a positive pulse occurring between a time T₀ and alater time T₂, followed by the burst interval shown containing abouteight cycles of continuous wave subcarrier burst signal V_(b).

Sync separator 30 provides an output sync pulse V_(s), derived from thevideo signal, of a positive polarity and with a peak amplitude, forexample, of 25 volts. Voltage divider resistors 52 and 53 attenuate syncpulse V_(s) to provide a positive input pulse V_(i) (FIG. 2) with adesired peak amplitude of 4 volts in this example.

Integrating network 56, 58 integrates pulse V_(i) to produce a rampvoltage waveform V_(B) (FIG. 3) at the base of transistor 60 betweentime T_(O) and time T₂. Integrating network 56, 58 also serves toenhance the noise-immune operation of gating circuit 50.

The values of resistor 56 and capacitor 58 determine an RC time constantsuch that, after a predetermined length of time, the magnitude of theramp voltage V_(B) at the base of transistor 60 reaches about +0.65volts. At this time, T₁, the base-emitter junction of transistor 60 issufficiently forward biased so that transistor 60 conducts. Thus,resistor 56 and capacitor 58 delay by a predetermined amount the time atwhich transistor 60 conducts in response to pulse V_(i) prior to theburst interval. It is noted that prior to time T₁ at which transistor 60conducts, resonant circuit capacitor 66 had charged to the operatingsupply voltage (+4 volts) via resistor 61.

FIG. 4 represents a collector voltage waveform V_(C) of transistor 60.The collector voltage V_(C) rapidly drops to substantially zero voltswhen transistor 60 conducts at time T₁. Transistor 60 essentially servesas a switch such that when rendered conductive at time T₁, resonantcircuit 65 is excited into ringing at its natural frequency. If is notedthat at this time capacitor 66 is effectively in parallel with inductor68 because of the low collector-emitter impedance of transistor 60 whenswitched into conduction. The ringing frequency of resonant circuit 65is determined by the time constant established by the values ofcapacitor 66 and inductor 68. In this example, these values are chosenso that the time of one-half of one ringing cycle is substantially equalto the time of the burst interval (e.g., about 5 microseconds).

FIG. 5 illustrates the current (I_(L)) of inductor 68 when resonantcircuit 65 is excited into ringing. FIG. 6 illustrates the relatedemitter current (I_(E)) of transistor 60 at the same time. Inductorcurrent I_(L) flows in a negative direction beginning at time T₁ andremains of a negative polarity for the first full one-half cycle ofringing until a time T₃. Emitter current I_(E) of transistor 60 iscoincident in time, but of an opposite polarity (positive), with respectto inductor current I_(L). The voltage developed by inductor 68, V_(L),leads the inductor current I_(L) by 90° as depicted by FIG. 8.

At time T₃, after one-half of one ringing cycle has been completed, thepolarity of inductor current I_(L) changes from negative to positive andcontinues to be of positive polarity until time T₄, as indicated by FIG.5. During this time period the polarity of the emitter current I_(E) oftransistor 60 changes from positive to negative as shown by FIG. 6. Thusduring the time period T₃ -T₄, emitter current I_(E) and inductorcurrent I_(L) flow in a direction opposite to that shown for thesecurrents in FIG. 1.

The roles of the collector and emitter of transistor 60 are effectivelyinterchanged for the latter condition described above such that theemitter current I_(E) of transistor 60 flows from the emitter to thecollector of transistor 60 from time T₃ to time T₄. Transistor 60continues to conduct from time T₃ to time T₄ but in an inverse currentconduction mode in which transistor 60 exhibits a common emitter forwardcurrent transfer ratio (h_(FE)) reduced by about an order of magnitude.

Prior to time T₃, the base voltage V_(B) of transistor 60 had begun todecrease when input pulse V_(i) dropped toward zero volts at time T₂.From T₃ to time T₄ during which emitter current I_(E) exhibits anegative polarity, the collector voltage V_(C) of transistor 60 exhibitsa negative polarity as shown by FIG. 4. During this time period the basevoltage V_(B) of transistor 60 may be considered to be pulled negativeby the negative-going collector voltage V_(C), as shown by FIG. 3, andbase current flows from the base to the collector of transistor 60 sincethe base-collector junction of transistor 60 remains forward biased byabout 0.65 volts. Transistor 60 therefore conducts negative currentrepresentative of the next half cycle of ringing between times T₃ andT₄. Also during this time, the charge on capacitor 58 continues to bedepleted due to the conduction of transistor 60. A portion of the chargeon capacitor 58 also is depleted via resistors 56 and 53.

As shown in FIGS. 5 and 6, at time T₄ inductor current I_(L) attempts tochange from positive to negative polarity and the related emittercurrent I_(E) correspondingly attempts to change from negative topositive polarity. That is, the emitter of transistor 60 attempts torevert to its original role of supplying current in the directionindicated by FIG. 1. At this time, however, the baseemitter junction oftransistor 60 is not sufficiently forward biased to maintain transistor60 in conduction because the charge on capacitor 58 has been depleted,and transistor 60 turns off at the end of substantially one ringingcycle.

In accordance with the operation described above, gating circuit 50produces an output burst gate pulse V₀ shown by FIG. 8. Output gatepulse V₀ is represented by the first full positive half cycle ofinductor voltage V_(L) which begins at T₂ and ends between T₃ and T₄. Ascan be seen from FIGS. 7 and 8, output gate pulse V₀ is coincident withthe burst interval of the video signal containing burst component V_(b).In this example, output gate pulse V₀ is provided to coincide with theburst interval by means of the time delay T_(O) -T₁ for delaying theinitial conduction of transistor 60, and by means of the time delay T₁-T₂ associated with the time of the first quarter of the first fullringing cycle of inductor voltage V_(L) (FIG. 8).

At time T₄ capacitor 66 begins to charge toward the operating supplyvoltage of 4 volts via resistor 61. In this context resistor 61 andcapacitor 66 form an RC charging network with a time constant determinedby the values of resistor 61 and capacitor 66 to be less than the timeof one horizontal time scanning period (e.g., 63.5 microseconds).Capacitor 66 charges to substantially the level of the operating voltagesupply by the end of the horizontal line scanning interval and prior tothe arrival of the next sync pulse V_(S). In connection with thefunction of resistor 61, it is also noted that a series resonant circuitcomprising resistor 61, capacitor 66 and inductor 68 is formed whentransistor 60 turns off at time T₄. Resistor 61 additionally serves todamp any tendency of the series resonant circuit so formed to ring atthis time.

Although the invention has been disclosed in terms of a particularcircuit embodiment, it should be appreciated that other arrangements maybe devised by those skilled in the art without departing from the scopeof the invention.

For example, the input pulse V_(S) may be a horizontal sync pulse, asshown, or a flyback pulse derived from the horizontal deflection circuitof the television receiver. The timing of output gate pulse V₀ may beadjusted (i.e., delayed) to coincide with the burst interval by varyingthe value of either or both of capacitor 66 and inductor 68 to adjustthe period of ringing of resonant circuit 65.

The time at which resonant circuit 65 is excited into ringing and hencethe timing of output gate burst V₀ can also be tailored by employingvarious input circuit arrangements to delay the time at which transistor60 initially conducts in response to pulse V_(i). Such circuitarrangements are not essential, however, since a suitable delay forproviding a properly timed output pulse V₀ can be achieved by adjustingthe period of ringing of resonant circuit 65 as previously mentioned.

Although the operation of transistor 60 has been described in connectionwith capacitor 58, this capacitor is not required, for example, wheninput pulse V_(S) is applied to the base-emitter circuit of transistor60 from a low impedance source. A sufficiently low impedance input pulsesource in combination with a ringing waveform of sufficient amplitudeproduced by resonant circuit 65 would enable the base of transistor 60to remain sufficiently forward biased with respect to the emitter andcollector of transistor 60 when transistor 60 is operating in theforward and inverse conduction modes during times T₁ -T₃ and T₃ -T₄,respectively, thereby permitting transistor 60 to remain conductiveduring one full ringing cycle T₁ -T₄ as explained.

Lastly, it is noted that an output gate pulse V₀ of negative polaritycan be produced by interchanging the relative positions of capacitor 66and inductor 68. In this case, the voltage and current amplituderesponse of resonant circuit 65 would be of opposite polarity to thoserespectively shown in FIGS. 5, 6 and 8 for the disclosed embodiment ofthe present invention.

What is claimed is:
 1. Apparatus for generating a burst gate pulsesuitable for use in processing color information contained in achrominance component of a color television video signal also having ahorizontal synchronizing component and a color reference burst componentoccurring during a burst interval, said apparatus comprising:a source ofoperating potential; input means for providing a pulse representative ofsaid horizontal synchronizing component; transistor switch means havingan inverse current conduction characteristic and an input electrodecoupled to said input means, an output electrode coupled to said sourceof operating potential, and a common electrode, said output and commonelectrodes defining a main current conduction path therebetween; andresonant circuit means comprising at least an inductance and a firstcapacitance, coupled across said main current conduction path of saidtransistor switch means and having a predetermined first time constant,said resonant circuit means being excited into ringing when saidtransistor switch means conducts in response to said pulse to produce aringing waveform with a period determined by said first time constantand with a duration determined by current conduction of said transistor,wherein said resonant circuit means coacts with said inverse conductioncharacteristic of said transistor to render said transistornon-conductive prior to exceeding a first full cycle of said waveform,to inhibit amplitude excursions of said waveform beyond said first fullcycle and to provide said waveform with an output pulse corresponding toa first full half cycle of one polarity of said waveform and coincidentwith said burst interval.
 2. Apparatus according to claim 1, whereinsaid inductance and said first capacitance are coupled in series acrosssaid main current conduction path of said transistor.
 3. Apparatusaccording to claim 2, wherein one end of said first capacitance remotefrom a point of interconnection of said inductance and first capacitanceis coupled to said output electrode, and one end of said inductanceremote from said point of interconnection is coupled to said commonelectrode.
 4. Apparatus for generating a burst gate pulse suitable foruse in processing color information contained in a chrominance componentof a color television video signal also having a horizontalsynchronizing component and a color reference burst component occurringduring a burst interval, said apparatus comprising:a source of operatingpotential; input means for providing a pulse representative of saidhorizontal synchronizing component; transistor switch means having aninput electrode coupled to said input means, an output electrode coupledto said source of operating potential, and a common electrode; andresonant circuit means comprising at least an inductance and a firstcapacitance coupled in series across a main current conduction path ofsaid transistor, one end of said first capacitance remote from a pointof interconnection of said inductance and said first capacitance beingcoupled to said output electrode and one end of said inductance remotefrom said point of interconnecting being coupled to said commonelectrode; said resonant circuit means having a predetermined first timeconstant and being excited into ringing when said transistor switchmeans conducts in response to said pulse to produce a ringing waveformwith a period determined by said first time constant, wherein saidresonant circuit means coacts with an inverse conduction characteristicof said transistor to render said transistor non-conductive prior tocompletion of a first full cycle of said waveform to provide saidwaveform with an output pulse corresponding to a first full half cycleof one polarity of said waveform and coincident with said burstinterval, said output pulse being derived from said point ofinterconnection of said inductance and said first capacitance means andcorresponding to a first full half cycle of positive polarity of avoltage developed across said inductance.
 5. Apparatus according toclaim 4 and further comprising:charge storage means coupled to saidinput electrode for developing a voltage representative of said inputpulse at said input electrode.
 6. Apparatus according to claim 5,wherein said charge storage means comprises a second capacitance andwherein said apparatus further comprises:a first resistance coupled tosaid second capacitance to form an integrating network therewith forintegrating said input pulse.
 7. Apparatus according to claim 6, whereinthe values of said first resistance and said second capacitancedetermined a second time constant for delaying by a predetermined amountthe time at which said voltage developed by said second capacitancereaches a magnitude sufficient to render said transistor conductive toproduce said ringing waveform.
 8. Apparatus according to claim 7,wherein said first time constant is substantially equal to the timeperiod of said burst interval.
 9. Apparatus according to claim 8,wherein a second resistance couples said output electrode of saidtransistor to said source of operating potential and wherein the valuesof said second resistance and said first capacitance determine a thirdtime constant such that said first capacitance charges to substantiallythe level of said operating potential via said second resistance aftersaid transistor is rendered non-conductive and prior to a nextsucceeding input pulse.
 10. Apparatus according to claim 9, wherein saidinput, output and common electrodes of said transistor respectivelycorrespond to base, collector and emitter electrodes, and saidtransistor is arranged in common emitter configuration.